The objective for the quality of operation of the network relating to the psophometric electromotive force corresponding to the noise due to magnetic induction and/or to the electrical influence of all the electrical lines acting on one or more parts of the chain of telephone lines which connect the subscriber station to the international center on which it depends should not exceed 1 millivolt, this value being determined at the “line” terminals of the subscriber station (considered at reception) on the assumption that the telephone installations telecommunications inserted in this chain are symmetrical with respect to the earth, this symmetry being as perfect as possible, in accordance with the most modern technique of construction of devices.